California Code, Vehicle Code - VEH § 21204

(a) A person operating a bicycle upon a highway shall not ride other than upon or astride a permanent and regular seat attached thereto, unless the bicycle was designed by the manufacturer to be ridden without a seat.

(b) An operator shall not allow a person riding as a passenger, and a person shall not ride as a passenger, on a bicycle upon a highway other than upon or astride a separate seat attached thereto. If the passenger is four years of age or younger, or weighs 40 pounds or less, the seat shall have adequate provision for retaining the passenger in place and for protecting the passenger from the moving parts of the bicycle.